In known non-contact web cleaners, clean air at high velocity is passed over the surface of the web to remove debris. It is also known to utilise electrostatic charges to neutralise the charge which may be attracting the debris or dirt to the surface of the web.
U.S. Pat. No. 2,920,987 describes an arrangement which can be used to clean the surface of a non-conducting material. A high alternating voltage supply is applied to the material by means of an electrode positioned adjacent one surface of the web. This alternating voltage damps out the static charge present on the surface of the material due to a previous operation, for example, a grinding operation. Arranged on the opposite side of the material to the electrode is a series of projections which contact the material. Particles on the material after the grinding operation, say, are removed due to the electrostatic effect produced by the electrode and the physical contact of the projections with the material itself. Loosened particles are removed from the surface of the material by suction.
Another arrangement in which alternating voltages are used to assist in the removal of particles from the surface of a web is described in U.S. Pat. No. 2,980,933. In this arrangement, the web is passed through two static electrostatic fields of opposite charge, an air stream being used to remove the particles loosened by the oppositely charged electrostatic fields.
U.S. Pat. No. 4,213,167 discloses an arrangement in which both surfaces of a web are cleaned by causing air to flow over both surfaces, and introducing ions into the air flow to neutralise static charge on the web. Other examples of the use of ionised air streams to remove dust are disclosed in U.S. Pat. Nos. 4,454,621 and 4,241,377.
U.S. Pat. No. 4,835,808 discloses an arrangement which utilises both suction and blowing to remove particles from the surface of a web. A field plate is also used to charge the web and an electrode is used to discharge the web thereby removing the particles. U.S. Pat. No. 3,536,528 also discloses the use of suction and blowing to remove particles from the surface of a web, but in this case, the charge on the web is neutralised after cleaning.
In arrangements which utilise air at high velocity to remove loosened particles from the surface of a web, there is a lower limit to the size of particle which can be removed. This limit is due to the thickness of the boundary layer produced over the surface of the web by the high velocity air stream. The thickness of the boundary layer can be reduced by increasing the air velocity, but a practical limit is approximately 30 mm on a smooth surface and more than this on a rough one.
French Patent Specification FR-A-2179707 discloses apparatus and method for cleaning glass sheets. Electrostatic charge is generated on the sheet by means of corona discharge as it passes between two electrode strips arranged transverse to the direction of motion of the glass sheets, each strip having a plurality of needle electrodes arranged along its length. These electrode strips are used to discharge particles adhering to the glass sheet and once discharged, the particles are removed using an airstream.
Web cleaners which use ions to discharge the electrostatic charges holding particles against the surface of the web are only effective at low web speeds. However, this may prevent particles being reattracted to the web after they have been loosened.
It is therefore an object of the present invention to provide improved web cleaning apparatus which produces sufficient force using alternating electrostatic fields to separate particles from the surface of the web, in the boundary layer region, so that they can be removed from the surface of the web.